This invention relates to novel energy curable inks in which non-reactive branched polyesters are used to reduce misting. The invention also relates to methods of reducing misting and/or dot gain in ink compositions by using non-reactive branched polyester resins.
The use of gelled or structured varnishes is very important in controlling a number of printing problems. Two important functions of the gel varnish are controlling misting and dot gain.
Misting occurs when small droplets of ink are expelled from printing nips due to high printing speeds and low ink structure. They are a health and clean-up problem. These droplets form an aerosol which stays in the air and coats equipment, and also may be inhaled by plant employees. A gelled or structured varnish controls this by giving the ink enough elasticity so that the ink is pulled back into the press, and small droplets are less likely to be formed and expelled.
Dot gain is the spreading of dots of ink on paper to produce poor image quality. Again, the gelled or structured varnish prevents this by adding enough structure to the ink to prevent easy spreading.
Conventional UV curable paste inks, customarily use gelled resins to impart higher structure and viscosity, and improve properties such as misting and dot gain. The gelled vehicles are produced from resins that are reacted with chelating agents such as aluminum alkoxides. These materials, known as gellants, form coordinate covalent and hydrogen bonds. The amount of each type of bonding varies with the gellant being used. However, there is a significant drawback to this method of imparting structure. Most of the diluents used in UV inks are reactive acrylate monomers, which are fairly polar. These polar solvents cause a breakdown in the hydrogen bonding structure when added to the gelled varnish. To compensate for this, the varnishes are typically gelled with a very high level of gellant, which results in inks with poor flow and transfer. These are relatively hard to handle due to their high structure. They also make the inks print poorly owing to poor water emulsification properties.
It has now been found that resins that are designed to provide structure through crosslinking and entanglements are much more robust and do not suffer a loss of structure when mixed with polar solvents. According to the present invention, branched polyesters are such polymers since they provide structure through a combination of branching and chain entanglements to mimic the rheological properties of a xe2x80x9cgelled varnish.xe2x80x9d While branched polyesters are known in the prior art, this invention provides for the first time their use in energy curable printing inks and coatings This invention is also provides formulations containing branched polyesters which are incorporated in a UV curable printing ink by design to control ink properties.
The present invention relates to an energy curable printing ink composition having low misting and dot gain properties. Specifically, the printing ink composition of the invention includes non-reactive highly branched polyester resins that are designed to provide structure through crosslinking and entanglements and thereby increase rheology and reduce misting.